Graphene has recently received significant attention due to its many attractive properties, including: chemical and electrochemical stability; electrical conductivity; and high theoretical surface area (e.g., 2630 m2/g). This theoretical surface area compares well to those of: carbon nanotubes (CNT) (e.g., 1315 m2/g); and commercially available activated carbon (typically, 500 m2/g). A common method of producing CNT is chemical vapor deposition (CVD), which has remained quite expensive despite several decades of research and development. In contrast, graphene can be prepared by a number of diverse routes, ranging from CVD to mechanical cleaving of graphite. In particular, thermochemical exfoliation of graphite powder has rapidly become a cost-effective method for large-scale production of graphene and graphene oxide nanosheets. With this method, the price of graphene and graphene oxide sheets is projected to be $50/kg over next several years. This projected price is comparable to that of electronic grades of activated carbon.
Graphene oxide nanosheets may be regarded as being graphene nanosheets with various functional groups, such as carboxylic acid and phenolic hydroxyl groups, attached to the edges or basal plane. Graphene oxide can be reduced to graphene. Aqueous dispersions of graphene oxide nanosheets are colloidally stable, a state generally attributed to electrostatic interactions resulting from the ionization of these functional groups. Due to the presence of oxidized functional groups, graphene oxide nanosheets are surface active. Graphene oxide can be easily reduced by thermal reduction, chemical reduction, or flash reduction to obtain reduced graphene oxide, a material that is comparable to graphene.
In the majority of current research and practical applications, the processing of graphene oxide nanosheets is based on filtration of graphene oxide dispersion through a membrane filter. Concerns raised by such filtration-based methods include:
(1) stacking of graphene oxide nanosheets due to van der Waals force, which renders a portion of the surface area inaccessible, thereby adversely affecting the electrochemical or electrical properties (e.g., capacitance) of the graphene oxide devices;
(2) re-stacking of graphene oxide nanosheets during application, such as, for example, in supercapacitors where irreversible loss of capacity during cycling occurs, and is likely due to re-stacking of nanosheets during charging and discharging operation; and
(3) environmental, safety and health (ESH) concerns associated with both the processing and the application of graphene oxide, since the material is cytotoxic in its nanoscale form.